This invention relates to compensating for polarisation mode dispersion, particularly across the signal spectrum of an optical signal of an optical communications system, and to an optical element for providing a variable differential delay primarily but not exclusively for use in such polarisation mode dispersion compensation.
Existing communications systems typically rely, for transmission over long distances, upon the use of nominally single mode optical fibres which carry optical signals and provide transmission of signal data at rates of 10 Gb/sec or more over distances of the order of 100 kilometers or greater. Although such fibres are nominally single mode, propagation of optical signals is generally characterised in such fibres by two orthogonally polarised HE11 modes for which slightly different group velocities exist in the presence of birefringence.
An optical fibre may be regarded as a concatenation of birefringent elements through which the optical signal propagates. As the different modes of the signal will propagate through the fibre at different velocities due to the differing refractive indices experienced by the modes, the modes will become temporally separated.
For a given span of optical fibre, the difference in transmission time for these two modes (often referred to as xe2x80x9cfastxe2x80x9d and xe2x80x9cslowxe2x80x9d modes to indicate the difference in final propagation velocities) is termed polarisation mode dispersion (PMD). FIG. 1 shows a schematic representation of this effect, with both the fast and slow modes 10, 12 of the optical signal being launched into the optical fibre at time t. Both modes propagate along the fibre, with the faster mode 10 arriving at the end of the fibre span at time T and the slower mode 12 arriving at time T+dt, where dt is the difference in transit time between the signals due to polarisation mode dispersion.
FIG. 2 shows a part of a typical optical communication system that might include a polarisation mode dispersion compensator. An optical fibre (1) provides a transmission path for propagation.
It is known from U.S. Pat. No. 5,473,457 to analyse a received optical signal in a manner which permits the received optical signal to be separated into fast and slow mode components, the fast mode component then being subject to a compensating delay by means of transmission of both components through a polarisation maintaining optical fibre of pre-determined length and high polarisation dispersion to provide a differential delay, albeit a fixed differential delay.
FIG. 2 shows a part of a typical optical communication system that might include a polarisation mode dispersion compensator. An optical fibre (1) provides a transmission path for propagation of an optical signal from a polarised light emitting transmitter (2) to a receiver (3). This transmission path includes an erbium doped optical fibre amplifier (4), and, adjacent the receiver (3), a PMD compensator (5).
The optical fibre (1) is a nominally circular symmetric single mode fibre extending over a substantial distance. Over a distance of this length the departures from perfect circular symmetry of that fibre, for example as a result of bending strain, are liable to be of sufficient magnitude for the fibre to function as a concatenation of birefringent elements of random relative orientation, as described above. Consequently, such optical fibres require compensation for polarisation mode dispersion. Moreover, the birefringence of the fibres will change due to such effects as heating and cooling of the fibre, and changes in strains imparted to the fibre (particularly in overhead optical cables and cables affected by maintenance crews or other human intervention). Obviously, such systems require a PMD compensator that is variable.
It is known from WO97/50185 to compensate for PMD by splitting the received optical signal at the receiver into two polarisation states and to apply switched delays of different length to the separated components, thereby providing a variable delay, albeit a delay that is not continuously and smoothly variable, and that also requires a relatively complex optical switching configuration.
The inventor of the present invention has previously disclosed in U.S. Pat. No. 4,953,939 the use of an optical fibre chirped Bragg grating reflector in combination with a directional coupler to introduce a delay which is wavelength dependent because the periodicity of the Bragg grating varies with position along the fibre, so that different wavelengths are reflected from different positions along the fibre. In pending U.S. patent application Ser. No. 09/135,967 filed Aug. 18, 1998 (granted as U.S. Pat. No. 6,271,952), which is incorporated herein by reference, the present inventor provides an optical element for providing a variable differential delay and an associated method for polarisation mode dispersion compensation by separating the optical signal into fast and slow mode components and utilising a chirped Bragg reflector to provide a variable delay.
All of the above techniques relate to first order polarisation mode dispersion, as indicated in FIG. 1. Each technique requires the use of polarisation state controllers, to align with and separate the different polarisation states, prior to providing a differential delay to the separate states. It would be advantageous to provide a PMD compensator that does not require such a polarisation state controller.
GB2184252 discloses how the use of squeezer elements applying stress to an optical fibre may be used to change the birefringence of the fibre to produce an optical state-of-polarisation modulator.
The differential group delay is dependent upon the wavelength of the optical signal. The mechanism by which the PMD value of a fibre is seen to vary across the signal spectrum is often referred to as higher order PMD. As the differential group delay (DGD) due to PMD is not a single value across the signal bandwidth, accurate PMD compensation cannot simply be achieved by introducing a single value of differential delay in opposition to the differential group delay induced by the PMD of the fibre (as is described in the above techniques). For low values of PMD, the dispersion does indeed approximate to a single value across the signal bandwidth, thus permitting the use of such simple compensator designs as described above. However, for larger PMD values there exists a need to provide an improved method of providing a continuously variable optical delay and for compensating for polarisation mode dispersion in optical fibres across the signal spectrum.
In one aspect, the present invention provides an apparatus for compensating for polarisation mode dispersion, the apparatus comprising a chirped Bragg reflector extending longitudinally along an optical waveguide, said waveguide being susceptible to stress birefringence; and at least one tuning means located at a position along the length of said waveguide, said tuning means being operable, in use, to apply a stress to said waveguide so as to alter the magnitude and the orientation of the birefringence of said waveguide for compensation of the polarisation mode dispersion of the optical signal.
It is well known that with low chirp fibre gratings, quite low levels of birefringence, can cause significant PMD. This invention exploits this fact and provides a means of compensating for differing PMD. This approach has the advantage over PMD compensation techniques described in the prior art, in so far as it does not require a separate polarisation state controller to align with the different states prior to applying different delays to the orthogonal states.
Preferably, the apparatus comprises at least two of said tuning means, each tuning means being located at a different position along the length of said waveguide. Advantageously, such an apparatus may be used to affect the birefringence of the waveguide so as to compensate for the polarisation mode dispersion caused by the birefringence of a system such as an optical transmission system. By applying stress at at least two distinct points along the length of such a chirped Bragg reflector, different parts of the signal spectrum will undergo different birefringent conditions dependent upon where they are reflected within the Bragg reflector and hence polarisation mode dispersion compensation may be achieved across the signal spectrum of the optical signal. Consequently, the present invention provides a means of compensating for differing PMD values for different parts of the signal spectrum i.e. it is arranged to provide PMD compensation across the signal spectrum of an optical signal.
Preferably said optical waveguide comprises a birefringent material arranged such that the magnitude and orientation of the birefringence is in use controlled by stress imparted by said tuning means. The optical waveguide may be formed completely or partially from a material that is birefringent. By applying a simple single stress by each tuning means, it is possible to adjust the orientation and relative magnitude of the birefringence of the fibre so as to achieve the desired compensatory effect. The stress could of course be a compressive or a tensile stress.
Preferably said tuning means are arranged such that in use the position of the tuning means in relation to the waveguide may be adjusted. The longitudinal position of the tuning means in relation to the fibre may be adjusted to affect different parts of the signal spectrum. The orientation of the tuning means about the axis defined by the waveguide may be adjusted (i.e. may be controllable) so as to provide the desired orientation of stress in order to achieve the required birefringent conditions of the waveguide for optimum PMD compensation. Similarly, the area around the perimeter of the optical waveguide and/or the longitudinal length of the optical waveguide to which the tuning means applies the stress may be adjustable so as to allow a variation in the longitudinal length of the waveguide which would be affected by the stress imparted by the tuning means. This would permit a variation in the wavelength and wavelength range of the optical signal spectrum affected by the tuning means.
Preferably at least one of said control means comprises two squeezing means in mutual proximity, and arranged such that in use said squeezing means can apply pressure to said waveguide along respective axes at a relative angle of xcex8 to each other, xcex8 being greater than zero, so as to provide a transverse stress for tuning of said birefringence.
By providing such squeezing means, and controlling the magnitude and relative magnitude of the stresses applied by the squeezing means, it is possible to control both the magnitude and the orientation of the stress induced birefringence, and hence the associated PMD compensation.
Preferably said squeezing means comprises electrical actuators such as electrostrictive or magnetostrictive devices or piezoelectric devices. Alternatively, selectively controlling the temperatures of different parts of the waveguide could be used to apply stresses to the waveguide for selectively controlling the waveguide birefringence.
Preferably, xcex8 is approximately 45xc2x0, the apparatus further comprising a third squeezing means, in mutual proximity with said first two squeezing means, arranged to apply pressure to said waveguide along an axis substantially perpendicular to one of said axes of said first two squeezing means.
Alternatively, the apparatus further comprises at least one longitudinal strain tuning means arranged such that in use a longitudinal tensile or compressive strain may be applied to at least a longitudinal section of said waveguide.
By applying either a longitudinal strain to a section of the waveguide, or orthogonal transverse strains to the waveguide, the chirp (and hence the associated reflection characteristics) may be adjusted. This might be used to compensate for variations in the waveguide characteristics or the variations in the telecommunications system associated with the PMD compensator due to ageing. By applying such strains, the grating bandpass may be adjusted to ensure alignment with the signal spectrum.
Alternatively, said optical waveguide may comprise an optical fibre, and at least one of said tuning means may comprise an actuator arranged to apply a variable bending moment to said fibre.
Preferably said Bragg reflector comprises a chirped sampled grating. This enables a single short grating to be used for any one of many different wavelength ranges (e.g. channels) as desired.
Preferably; the apparatus further comprises a control means for control of said tuning means, said control means being arranged to, in use, successively optimise the birefringence conditions of a pre-determined bandwidth of said optical signal.
Preferably wherein said optimisation is selected from maximising a monitor of the received eye opening, minimising the bit error rate or optimising the electrical spectrum of the return path of the optical signal from the apparatus. Each tuning means or part thereof (e.g. a squeezing element) might be sequentially adjusted so as to provide PMD compensation. Of course, if the appropriate control information signals were available relating to the different parts of the signal spectrum, all of the desired tuning means might be simultaneously adjusted. For instance, known techniques such as those disclosed in EP-0863626 could be utilised to determine the polarisation mode dispersion.
In another aspect, the present invention provides a telecommunication system comprising an apparatus as described above.
In a further aspect, the present invention provides a telecommunication system as described above, wherein the apparatus is being used to provide compensation for polarisation mode dispersion to an optical signal.
In a further aspect, the present invention provides a method for compensating for polarisation mode dispersion across the signal spectrum of an optical signal, the method comprising transmitting an optical signal along an optical waveguide which is susceptible to stress birefringence, and which has a chirped Bragg reflector extending longitudinally along said waveguide;
measuring the optical signal so as to provide an indication of the polarisation mode dispersion; and
applying stress to said waveguide at a position along the length of said waveguide so as to adjust the waveguide birefringence for compensation of polarisation mode dispersion of the signal across the signal spectrum.
Preferably, the optical signal is measured so as to provide an indication of the polarisation mode dispersion across the signal bandwidth; and said stress is applied at at least two distinct positions along the length of said waveguide so as to adjust the waveguide birefringence for compensation of polarisation mode dispersion of the signal across the signal spectrum.
Preferably, said measurement of the optical signal is selected from maximising a monitor of the received eye opening, minimising a bit error rate or optimising the electrical spectrum of the optical signal reflected from the Bragg reflector.
The invention also provides for a system for the purposes of polarisation mode dispersion compensation which comprises one or more instances of apparatus embodying the present invention, together with other additional apparatus.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.